At this point, you almost have to feel sorry for the Christian right. The movement’s most famous leaders are dead or fading from view. It’s stuck with a presidential candidate who barely goes to church. It’s losing gay marriage court cases left and right. Yet still, its ideological opponents are bent on discovering new corners of American life that conservative Christians have single-handedly destroyed.

Dagmar Herzog, a history professor at the Graduate Center of the City University of New York, tries to re-stoke the culture wars with “Sex in Crisis,” a chronicle of the Christian right’s supposed influence in the American bedroom. Herzog opens with a depressing snapshot of the modern marriage: Americans work too hard and spend too much time with kids to bother with romance in the evening. The easy availability of Viagra has recast sex as a male-centered mechanical act. Husbands often prefer Internet porn to sleeping with their wives. (“Not Tonight, Honey, I’m Logging On,” reads one headline she quotes.) One in three women and one in six men, according to one study, had experienced loss of desire for sex. “Americans,” Herzog writes, have “lost their libidos.”

Into this barren landscape steps the Christian right, ready to take over “the terms of the conversation about sex and love in the United States.” Shedding their frigid stereotype, Christian leaders have embraced a new role as sex therapists to a love-starved nation. Sex can be fabulous, orgasmic, even kinky, these new gurus promise — but only in the confines of a heterosexual, Christian marriage. Stray from that path, and you will find yourself bitter, estranged and diseased.

For the anthropologically minded, few activities are as entertaining and rewarding as digging through Christian sex advice books, which tend to be loaded with forced enthusiasm and awkward metaphors. Herzog, a pastor’s daughter who grew up in what she calls a “deeply Christian household,” starts by going back to the early 1970s and Tim LaHaye’s handbook “The Act of Marriage,” which calls sex “life’s most exciting experience” and extols the “titanic emotional and physical explosion that culminates the act of marriage.” A more modern book streamlines the cheerleading: “What an incredible thought! Passionate sex was God’s idea.” Another manual compares masturbation to “slaking your parched thirst with salt water.” Instead, men are told to eliminate the “junk sex” of cable TV movies and lingerie ads so they can crave “real food” — i.e., their wives.

Herzog’s best observation is about the creepiness of what she calls “the Christian pornography.” Like Daniel Defoe warning about the criminal life in “Moll Flanders,” the Christian sex authors dig lustily into the fine points of depravity in the service of their morality tale. In one book, “Alex” sees his sister-in-law asleep in front of the TV, catches a glimpse of her upper thigh and a “trace of underwear” and finds himself masturbating while she sleeps, “right out in the open.” A prominent pastor confesses his desire to masturbate in his car after a transaction with a “lovely bank teller.” (A.T.M., anyone?) In Herzog’s most disturbing example, a manual tells of a married father of three working with a youth group who drives a 15-year-old home and winds up having sex with her in a park. These stories are offered as warnings about what happens when you stray from the marital model, but they also double as sanctioned soft porn.

Herzog’s larger arguments, however, are very familiar and not terribly nuanced. In chronicling the fight against gay marriage, Herzog tends to reduce the Christian right position to its most extreme exemplars, trotting out Lou Sheldon and Donald Wildmon — two dinosaurs of the movement — for some reliably outrageous quotations. She would have done better to devote more space to tracing the evolution of the anti-gay position through speeches and writing, just as she did with sex advice. She might have pinpointed when and how activists overstepped and found themselves in a position where courts and state legislatures — not to mention the general public — seem increasingly open to civil unions, and even gay marriage.

Herzog describes how the Christian right, with the support of the Bush administration, has pushed abstinence on teenagers.But she doesn’t note that as the evidence against abstinence education piles up, schools are increasingly resisting it.

Herzog’s most convincing chapter describes the Christian right’s influence abroad. By lobbying the Bush administration, and setting up a vast network of missions, the movement has managed to spread its vision of sexual purity into the developing world. Herzog documents how the abstinence-first policy has severely reduced the supply of condoms in places where they can be a critical tool in fighting AIDS. But even here, Herzog refuses to acknowledge any shades of gray. She criticizes Rick Warren, the most famous of the new generation of pastors, for pushing faith as a solution to the crisis. But Warren has also said he’s not against other people’s distributing condoms. Herzog cites this as an example of “hedging,” but it’s actually a bold break with orthodoxy that you would think she would applaud.

Herzog’s argument suffers in part because she declines to define exactly who the “Christian right” is. The big, bad Christian right of the early ’90s no longer exists. Many Americans casually call themselves “evangelicals,” but on measures of sexual purity — premarital sex, divorce, use of pornography — these “evangelicals” tend to poll just like the rest of America.

There is a “Christian right” as Herzog seems to intend it. These are the minority of evangelicals who attend church at least weekly. These are the people who do save themselves until marriage, who do believe disease and heartache follow naturally from premarital sex. They believe in patriarchy and female submission and an abundance of children. No doubt Herzog will be dismayed to learn that in national surveys, the wives in such marriages say they are happier and have more orgasms than the average American woman. But they, their husbands and their pastors do not dictate sexual mores in the rest of the country.

In 15 years, Herzog writes, the Christian right has “managed to undo the most important achievements of the sexual revolution.”But Herzog’s friends trapped in sexless marriages are not turning to Tim LaHaye for help. They are talking to their shrinks, or watching HBO’s “Tell Me You Love Me” for clues about what’s normal. The national conversation about sex and love is not dominated by Lou Sheldon or Donald Wildmon or even Rick Warren. It’s dominated by Carrie Bradshaw and Us magazine and Nerve.com.

Herzog laments that the United States is not Europe, where teenage sex is considered natural and beautiful (or at least a subject for long, lugubrious coming-of-age movies). But this has long been a puritan country, where sex comes loaded with guilt. Only these days, the problem is not so much teenagers being manipulated by their youth pastors. Instead, it’s the hookup culture and Miley Cyrus posing almost naked and average 16-year-olds freely sharing the details of their sex lives on MySpace.

* Hanna Rosin is the author of “God’s Harvard: A Christian College on a Mission to Save America.”